The invention relates to an apparatus for measuring film bubbles, comprising a guide track that extends in circumferential direction of the film bubble with a constant spacing relative to the latter, said guide track guiding a carriage which carries a measuring head facing the peripheral surface of the film bubble.
In the art of film blowing, it is desirable to perform measurements on the film bubble during the production process. In particular, it is desired to continuously monitor the thickness distribution of the film over the periphery of the film bubble, so that the cooling and/or extrusion temperature may be feedback-controlled for obtaining a constant thickness profile. In this case, the measuring head may for example be a capacitive thickness gauge, that hovers on an air cushion over the outer peripheral surface of the film bubble as has been described in WO 2009/027037.
EP 1 674 821 A1 describes a measuring device of the type indicated above wherein two carriages are guided on the guide track, and these carriages are connected to one another by a beam. The measuring head is mounted in the center of the beam. The beam consists of two parts that are pivotally or telescopically connected to one another, so that the distance between the two carriages on the guide track may be varied and, consequently, the radial position of the measuring head may be adapted to the actual diameter of the film bubble, while the measuring head stays facing the peripheral surface of the film bubble.